The Effect of Major Design Parameters on the Performance of a Downhole Coaxial Heat Exchanger

Studies on the Downhole Coaxial Heat Exchanger (1st Report)

Abstract

A downhole coaxial heat exchanger has as an advantage its simplicity in configuration. Furthermore, it is applicable to a wide variety of geothermal resources such as Hot Dry Rocks, magma bodies and geothermal reservoirs which need artificial lift for production.
In order to find an optimum design for the downhole coaxial heat exchanger, we have examined the effect of several design parameters on the performance of the heat exchanger by using a numerical simulator. The parameters that were treated as variable include circulation mode, thermal conductivity of the inner pipe, diameters of the wellbore and the inner pipe.
The following results were obtained:
1) Reverse circulation is suitable for attaining maximum thermal output, because the length over which heat loss occurs along the borehole is shorter for reverse circulation than for forward circulation.
2) Much higher outlet water temperature and net thermal output can be obtained by using an insulated inner pipe as opposed to a conventional steel pipe. The improvement ratio of net thermal output amounts to 10.3 for reverse circulation and 8 for forward circulation, assuming thermal conductivity of the inner pipe is 0.01 kcal/mh°C and water flow rate is 0.3 m³/min.
3) Due tq difference in water density, a circulation pressure arises in the downhole coaxial heat exchanger. No pumping work is needed for circulation when temperature differences between inlet water and outlet water are sufficiently large and the design of the heat exchanger is appropriate.
4) Considering the result of the experiments (Pan et al., 1982; Freeston and Pan, 1983), it can be considered that thermal output of a downhole coaxial heat exchanger with an insulated inner pipe and reverse circulation is much greater than that of a U-tube heat exchanger.
5) Thermal output increases with increasing diameter of the wellbore when reverse circulation is adopted, as it was shown for forward circulation by Horne (1980). However, only an 11% increase in thermal output is attained for a diameter increment of 62% with a certain condition specified in this work. Economically, it is advisable not to make the wellbore diameter larger, but to drill more wells of small diameters to increase thermal output.
6) Although the dependence of thermal output on the inner pipe diameter is much less than that on the diameter of the wellbore, friction loss and Pout-Pin are very much sensitive to the inner pipe diameter.
7) There is a value for inner pipe diameter which gives a minimum thermal output, if other parameters are kept the same. At a certain value of inner pipe diameter the total cost per output energy becomes minimum, if we consider the cost of pumping work needed to circulate water and the cost of the inner pipe.
8) Friction loss can be minimized with the inner pipe diameter at 11.7 cm for a case in which the diameter of the wellbore is 21.6 cm and the wall thickness of the inner pipe is 1.9 cm. At this time, the water velocity ratio between the inner pipe and annulus is 1.64:1 and Reynolds Number ratio between them is 3.16:1.